1. Field of the Invention
The present invention relates to a method, an apparatus and a program for radiation imaging, which are used for constituting an image on the basis of radiation image information obtained by radiation imaging. In this application, the word xe2x80x9cradiationxe2x80x9d is used in a wide sense so as to include a corpuscular beam such as an electron beam, or an electromagnetic wave, in addition to a general radiation such as X-rays, xcex1-rays, xcex2-rays, xcex3-rays, ultraviolet rays and the like.
2. Description of a Related Art
Conventionally, an imaging method using X-rays or the like is utilized in various fields, and employed as one of the most important means for diagnosis, particularly, in a medical field. Since a first X-ray photograph was realized, X-ray photography has been repeatedly improved and a method using a combination of a fluorescent screen and an X-ray film is predominantly used at present. On the other hand, in recent years, various digitized devices such as X-ray CT, ultrasonic or MRI are in practical use and establishment of a diagnostic information processing system and the like in hospitals is being promoted. As for X-ray images, many studies have also been made for digitizing an imaging system. The digitization of the imaging system not only enables a long-term preservation of a large amount of data without incurring deterioration in image quality but also contributes to development into the medical diagnostic information system.
Incidentally, a radiation image obtained as described above is generated by converting intensity of radiation transmitted through an object into brightness of the image. For example, in the case of imaging a region including a bone part, the radiation transmitted through the bone part is largely attenuated, and the radiation transmitted through a region other than the bone part, namely, a soft part is slightly attenuated. In this case, since the difference in the intensity of the radiation transmitted through different tissues is large, the radiation image with high contrast can be obtained.
On the other hand, for example, in the case of imaging a region of the soft part such as a breast, since the radiation is easily transmitted through the soft part as a whole, the difference between tissues in the soft part hardly appears as the difference in the intensity of the transmitted radiation. Because of this, as for the soft part, only a radiation image with low contrast can be obtained. Thus, the radiation imaging method is not suitable as a method of visualizing slight difference between tissues in the soft part.
Herein, information contained in radiation transmitted through an object includes phase information in addition to intensity information. In recent years, a phase contrast method has been studied in which an image is generated by using the phase information. The phase contrast method is an image construction technique for converting the phase difference resulted by transmitting X-rays or the like through the object into the brightness of the image.
Examples of the phase contrast method include a method of obtaining the phase difference on the basis of interference light generated by using an interferometer or a zone plate, and a method of obtaining the phase difference on the basis of diffracted light. Among them, in the method of obtaining the phase difference on the basis of the diffracted light, which method is called as a diffraction method, the phase difference is obtained on the basis of the following principle. For example, X-ray propagates through substance by travel of waves similar to light. Propagation velocity thereof varies depending on a refractive index of the substance. Therefore, when irradiating an object with an X-ray that has a uniform phase, the way the X-ray propagates varies in accordance with the difference between tissues in the object. For this reason, a wave front of the X-ray transmitted through the object is distorted and, as a result, diffraction fringes are produced on an X-ray image obtained on the basis of the transmitted X-ray. A pattern of the diffraction fringes varies depending on the distance between a screen on which the X-ray image is formed and the object, or wavelength of the X-ray. Accordingly, by analyzing two or more sheets of X-ray images having different diffraction fringe patterns, phase difference of X-rays, which is produced at each position of the screen, can be obtained. By converting the phase difference into the brightness, the X-ray image, in which difference between tissues in the object clearly appears, can be obtained.
In particular, in the radiation transmitted through a soft part of an object, the phase difference is larger than the intensity difference depending on the difference of tissues through which the radiation has transmitted. Therefore, delicate difference between tissues can be visualized by using the phase contrast method.
For the purpose of using such a phase contrast method, imaging conditions in performing the radiation imaging or techniques for restoring the phase from the diffraction fringe patterns are being studied. For example, T. E. Gureyev et al. xe2x80x9cQuantitative In-Line Phase-Contrast Imaging with Multienergy X Raysxe2x80x9d, PHYSICAL REVIEW LETTERS Vol. 86, No. 25 (2001), pp. 5827-5830 discloses that the phase restoration is performed on the basis of image information obtained by X-ray imaging with three types of X-rays having different wavelength respectively.
In the reference, relationship between phase and intensity of the X-ray just after having transmitted through an object to be inspected, and intensity of the X-ray at a predetermined distance from the object is noticed. That is, in the reference, as shown in FIG. 8, the X-ray imaging is performed on the assumption of such configuration that three types of X-rays having wavelength of xcex0, xcex1 and xcex2 respectively transmit through an object 100 to be inspected and enter a screen 102 disposed at a distance of R from an object plane 101.
In this case, relationship represented by the following expression stands up between intensity I(rxe2x8axa5,0,xcex0) and phase xcfx86(xe2x8axa5,0,xcex0) of the X-ray just after having transmitted through the object 100 to be inspected, and intensity I(rxe2x8axa5,R,xcexm) of the X-ray diffraction light detected on the screen 102, provided that in the following expression (1), I(rxe2x8axa5,0,xcex0)=exp{xe2x88x92M(rxe2x8axa5,0,xcex0)}                              A          ⁡                      (                                                                                M                    ⁡                                          (                                                                        r                          ⊥                                                ,                        0                        ,                                                  λ                          0                                                                    )                                                                                                                                        -                                                                  ∇                        2                                            ⁢                                              φ                        ⁡                                                  (                                                                                    r                              ⊥                                                        ,                            0                            ,                                                          λ                              0                                                                                )                                                                                                                                                                                                            ∇                      M                                        ·                                          ∇                                              φ                        ⁡                                                  (                                                                                    r                              ⊥                                                        ,                            0                            ,                                                          λ                              0                                                                                )                                                                                                                                          )                          =                  (                                                                      g                  0                                                                                                      g                  1                                                                                                      g                  2                                                              )                                    (        1        )            
where   A  =      (                                        -            1                                                γ            0                                                γ            0                                                            -                          σ              1              3                                                                          σ              1                        ⁢                          γ              1                                                                          σ              1              4                        ⁢                          λ              1                                                                        -                          σ              2              3                                                                          σ              2                        ⁢                          γ              2                                                                          σ              2              4                        ⁢                          γ              1                                            )  
provided that             σ      m        =                  λ        m                    λ        0              ,            γ      m        =                  R        ⁢                  xe2x80x83                ⁢                  λ          m                            2        ⁢                  xe2x80x83                ⁢        π              ,xe2x80x83gm=ln{I(rxe2x8axa5,R,xcexm) (m=0,1,2)
In the expression (1), when ∇Mxc2x7∇xcfx86(rxe2x8axa5,0,xcex0) is sufficiently small, it can be approximated as follows.                                           (                                                                                -                    1                                                                                        γ                    0                                                                                                                    -                                          σ                      1                      3                                                                                                                                  σ                      1                                        ⁢                                          γ                      1                                                                                            )                    ⁢                      (                                                                                M                    ⁡                                          (                                                                        r                          ⊥                                                ,                        0                        ,                                                  λ                          0                                                                    )                                                                                                                                        -                                                                  ∇                        2                                            ⁢                                              φ                        ⁡                                                  (                                                                                    r                              ⊥                                                        ,                            0                            ,                                                          λ                              0                                                                                )                                                                                                                                          )                          =                  (                                                                      g                  0                                                                                                      g                  1                                                              )                                    (        2        )            
Further, from the expression (2), the intensity and the phase of the X-ray just after having transmitted through the object 100 to be inspected are represented as follows.                               M          ⁡                      (                                          r                ⊥                            ,              0              ,                              λ                0                                      )                          =                                            λ              0                                      Δ              ⁢                              xe2x80x83                            ⁢              λ                                ⁢                      (                                          g                0                            -                                                σ                                      -                    2                                                  ⁢                                  g                  1                                                      )                                              (        3        )                                          -                                    ∇              2                        ⁢                          φ              ⁡                              (                                                      r                    ⊥                                    ,                  0                  ,                                      λ                    0                                                  )                                                    =                                            2              ⁢                              xe2x80x83                            ⁢              π                                      R              ⁢                              xe2x80x83                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              λ                                ⁢                      (                                          σ                ⁢                                  xe2x80x83                                ⁢                                  g                  0                                            -                                                σ                                      -                    2                                                  ⁢                                  g                  1                                                      )                                              (        4        )            
By performing an inverse Laplace operation on Laplace ∇xcfx862(rxe2x8axa5,0,xcex0) of phase in the expression (4), the phase xcfx86(rxe2x8axa5,0,xcex0)can be obtained. Further, by converting the phase into brightness of the image, a visible image representing the object can be obtained. By utilizing the expression (4) as just described, an operation for restoring phase can be simply performed on the basis of a small number of radiation images obtained while changing wavelength.
However, when phase is restored on the basis of about two or three radiation images, there arises a problem that, in the case where image quality is deteriorated due to influence of noise or the like, accuracy of the phase restoration deteriorates. In order to elevate the accuracy of phase restoration, it is considered to increase the number of radiation images by imaging while changing wavelength of the radiation applied to the object, however, when the wavelength is changed by using one radiation source, it takes a long time whenever the wavelength is changed. Further, when a plurality of radiation sources generating radiation different in wavelength are used, an apparatus becomes large size. On the other hand, it is also considered to increase the number of radiation images by imaging while changing not wavelength of radiation, but distance between the object plane 101 and the screen 102. However, there arises a problem that the apparatus becomes large size because moving distance of the screen 102 increases.
The present invention has been accomplished to solve the above-mentioned problems. A first object of the present invention is to provide a radiation imaging method capable of efficiently performing the imaging within a short period of time, as well as performing phase restoration with high estimation accuracy upon constituting a radiation image by using a phase contrast method. A second object of the present invention is to provide a radiation imaging apparatus for performing such a radiation imaging and a radiation imaging program for allowing a CPU to execute such a radiation imaging.
In order to solve the above-mentioned problems, a radiation imaging method according to the present invention comprises the steps of: (a) sequentially detecting, by using radiation having a first wavelength, intensity of radiation transmitted through an object on a plurality of planes different in distance from the object in a first order of increasing or decreasing the distance from the object so as to obtain a first group of image signals representing radiation image information on the plurality of planes respectively; (b) sequentially detecting, by using radiation having a second wavelength different from the first wavelength, intensity of radiation transmitted through the object on a plurality of planes different in distance from the object in a second order reverse to the first order so as to obtain a second group of image signals representing radiation image information on the plurality of planes respectively; (c) restoring phase information of the radiation transmitted through the object on the basis of the first group of image signals and the second group of image signals so as to obtain plural pieces of phase data; and (d) generating image data on the basis of the plural pieces of phase data obtained at step (c).
A radiation imaging apparatus according to the present invention comprises: a variable wavelength radiation source capable of generating radiation having a first wavelength and radiation having a second wavelength different from the first wavelength; detection means for detecting intensity of radiation transmitted through an object so as to obtain an image signal representing radiation image information; driving means to be used for altering distance between the object and the detection means; control means for controlling the variable wavelength radiation source and the driving means in such a manner that the detection means sequentially detects, by using radiation having a first wavelength, intensity on a plurality of planes different in distance from the object in a first order of increasing or decreasing the distance from the object, and then, the detection means sequentially detects, by using radiation having a second wavelength, intensity on a plurality of planes different in distance from the object in a second order reverse to the first order; and image constructing means for restoring phase information of the radiation transmitted through the object on the basis of a plurality of image signals obtained by disposing the detection means at a plurality of positions different in distance from the object so as to obtain plural pieces of phase data, and generating image data on the basis of the plural pieces of phase data.
A radiation imaging program according to the present invention actuates a CPU to execute the procedures of: (a) sequentially detecting, by using radiation having a first wavelength, intensity of radiation transmitted through an object on a plurality of planes different in distance from the object in a first order of increasing or decreasing the distance from the object so as to obtain a first group of image signals representing radiation image information on the plurality of planes respectively; (b) sequentially detecting, by using radiation having a second wavelength different from the first wavelength, intensity of radiation transmitted through the object on a plurality of planes different in distance from the object in a second order reverse to the first order so as to obtain a second group of image signals representing radiation image information on the plurality of planes respectively; (c) restoring phase information of the radiation transmitted through the object on the basis of the first group of image signals and the second group of image signals so as to obtain plural pieces of phase data; and (d) generating image data on the basis of the plural pieces of phase data obtained in the procedure (c).
In this application, the phrase xe2x80x9cradiation having a first wavelength or a second wavelengthxe2x80x9d is used so as to include not only radiation having a single wavelength but also radiation which has radiation intensity distribution in a plurality of wavelengths containing the first wavelength or the second wavelength as the central wavelength and which is regarded as radiation having substantially a single wavelength because of a narrow width of the radiation intensity distribution.
According to the present invention, a plurality of imaging are performed at a plurality of places on an outward route and an inward route by using radiation having different wavelength respectively, and therefore, a plurality of image signals having different radiation wavelength or distance between an object to be inspected and an imaging plane can be efficiently obtained within a short time. Further, phase information about the radiation transmitted through the object is restored on the basis of these image signals to generate image data, and therefore, high quality image data reduced in noise can be obtained.